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Ch 32: AC Circuits
Knight Calc - Physics for Scientists and Engineers 5th Edition
Knight Calc5th EditionPhysics for Scientists and EngineersISBN: 9780137344796Not the one you use?Change textbook
Chapter 32, Problem 28

A series RLC circuit has a 200 kHz resonance frequency. What is the resonance frequency if the capacitor value is doubled and, at the same time, the inductor value is halved?

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Step 1: Recall the formula for the resonance frequency of an RLC circuit: \( f_r = \frac{1}{2\pi\sqrt{LC}} \), where \( L \) is the inductance and \( C \) is the capacitance.
Step 2: Identify the initial resonance frequency \( f_r \) given in the problem as 200 kHz, and note the changes to the circuit components: the capacitance \( C \) is doubled (\( C' = 2C \)) and the inductance \( L \) is halved (\( L' = \frac{L}{2} \)).
Step 3: Substitute the new values of \( L' \) and \( C' \) into the resonance frequency formula: \( f_r' = \frac{1}{2\pi\sqrt{L'C'}} = \frac{1}{2\pi\sqrt{\frac{L}{2} \cdot 2C}} \).
Step 4: Simplify the expression inside the square root: \( \sqrt{\frac{L}{2} \cdot 2C} = \sqrt{LC} \). This shows that the resonance frequency remains unchanged.
Step 5: Conclude that the resonance frequency of the circuit is still 200 kHz, as the changes to \( L \) and \( C \) cancel each other out in the formula.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Resonance Frequency in RLC Circuits

The resonance frequency of a series RLC circuit is the frequency at which the inductive reactance equals the capacitive reactance, resulting in maximum current flow. It is given by the formula f₀ = 1 / (2π√(LC)), where L is the inductance and C is the capacitance. At resonance, the impedance of the circuit is minimized, and the circuit can oscillate freely.
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Effect of Capacitance on Resonance Frequency

Doubling the capacitance in an RLC circuit affects the resonance frequency inversely. According to the resonance frequency formula, increasing capacitance decreases the resonance frequency, as it is in the denominator of the square root. This means that if the capacitance is doubled, the resonance frequency will decrease, leading to a lower frequency of oscillation.
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Effect of Inductance on Resonance Frequency

Halving the inductance in an RLC circuit also influences the resonance frequency, but in the opposite direction. Since inductance is also in the denominator of the square root in the resonance frequency formula, reducing the inductance increases the resonance frequency. Therefore, if the inductance is halved, the resonance frequency will increase, resulting in a higher frequency of oscillation.
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